Elsevier

Desalination

Volume 347, 15 August 2014, Pages 131-137
Desalination

An experimental investigation on a solar still with an integrated flat plate collector

https://doi.org/10.1016/j.desal.2014.05.029Get rights and content

Highlights

  • Flat plate collector basin and conventional stills were fabricated and tested.

  • A horizontal flat plate collector (FPC) is integrated into the basin of the still.

  • FPC arrangement considerably enhances the distillate of the still.

  • The FPC basin still has about 60% higher distillate than the conventional still.

Abstract

This work promotes the performance of the single basin solar still by means of preheating the saline water using an integrated flat plate collector arrangement. A conventional single slope single basin still and a single slope flat plate collector basin still (FPCB still) are fabricated with the same basin area of 1 m2. The FPCB still is fabricated similar to a conventional still, with the integration of a horizontal flat plate collector arrangement to form six small compartments in the basin. The projected space between the consecutive basins acts as an extended surface which increases the temperature of the basin as well as the flat plate collector where the saline water is preheated before it enters the basin. Due to separate compartments (absorber plate) in the basin, the mass of water reduces and the evaporation rate increases for the same depth of water in the basin. Experiments are carried out by varying the water depth in the basin and using the wick and energy storing materials in basins of both stills. The FPCB still gives about 60% higher distillate than the conventional still for the same basin condition. Economic analysis shows that the cost of distilled water for the FPCB still is lower than that for the conventional still.

Introduction

A single basin solar still is a simple device used for desalination purposes. The productivity of a simple solar still is low. The performance of the solar still depends on various factors, such as: solar intensity, wind velocity, ambient temperature, water–glass temperature difference, free surface area of water, absorber plate area, temperature of inlet water, transparent cover angle and depth of water [1]. Several modifications were made in the solar still to improve its productivity. Reducing the water depth in the basin enhances the daytime productivity and reduces the nocturnal productivity [2], [3], [4], [5], [6]. Placing of energy storing materials [glass, black rubber, gravel, asphalt, quartzite rock, red brick pieces, cement concrete pieces, washed stones and iron scraps] in the basin improves the heat storing capacity and results in higher productivity of the still [7], [8], [9], [10], [11]. The latent heat thermal energy storage systems have many advantages over sensible heat storage systems including a large energy storage capacity per unit volume and almost constant temperature for charging and discharging [12]. In a single slope single basin solar still, a thin layer of stearic acid was used as a latent heat energy storage material in the basin [13]. The evaporation rate of a still strongly depends on the surface area of water exposed to the sun. The surface area exposure of the water can be increased by providing sponge cubes in the basin [14], [15].

In stills, evaporated water is condensed on the inside surface of the glass cover that releases the latent heat energy to the surroundings. It may be effectively used by providing an additional basin to the still, referred as a multi-effect still. Different methods to improve the performance of the multi-effect solar still were reviewed by Rajaseenivasan et al. [16]. Active methods significantly improve the temperature of water in the basin by integrating the still with external heat sources. A detailed review on the active solar distillation was made by Sampathkumar et al. [17]. Performance of the solar still integrated with a flat plate collector was studied using the tap and saline water [18]. Performance of the double slope solar still with a flat plate collector under natural circulation mode was investigated by Dwivedi and Tiwari [19]. A double effect solar still with integrated flat plate collector and water flow over the transparent cover was presented by Kumar and Tiwari [20].

Desalination system coupled with the parabolic collector with heat exchanger was theoretically and experimentally studied by Abdel Rehim and Lasheen [21]. An experimental analysis on a double slope solar still with non-tracking cylindrical parabolic concentrator with an electrical pump was presented [22]. The concentrator coupled still resulted in higher production than the passive and flat plate collector stills at various depth conditions.

Kumar and Tiwari [23] conducted an experimental study on a hybrid Photovoltaic/Thermal (PV/T) active solar still. Life cycle cost analysis of a single slope hybrid (PV/T) active solar still was presented by Kumar and Tiwari [24]. A solar still integrated with a pulsating heat pipe collector (PHP) was investigated by Sharif Abad et al. [25] with various filling ratios in PHP, various water depths in the still and various inclination angles of the flat plate collector. A double basin still integrated with a vacuum tube collector was used to enhance the distillate of the still by Panchal [26]. The productivity of this system was 56% higher than that of the conventional double basin still. A study was conducted in a single slope single basin still integrated with the solar water heater by Sampathkumar and Senthilkumar [27].

The research works described above clearly show that the performance of a still depends on mass of water in the basin, water temperature and exposure area. In active methods, a separate energy collector is used to supply the preheated water to the still. It raises the overall cost of the system as well as requires isolated space. The objective of this work is to accommodate the flat plate collector system into the single basin still, to enhance the distillate of the still by supplying the preheated water. It evades the additional space of the collector and reduces the collector system cost. In this work a horizontal flat plate collector is integrated in the basin of a single slope single basin solar still. This flat plate collector basin still (FPCB still) has the following advantages compare with the conventional basin still. (i) The flat plate collector arrangement provides preheated saline water supply to the basin that increases the basin water temperature. (ii) The fin arrangement increases the heat transfer rate from the basin to water. (iii) Due to separate compartments in the basin, the mass of water reduces and the evaporation rate increases for the same depth of water in the basin.

The experiments are conducted by varying the water depth in the basin, using black gravels and wick materials in the basin of conventional and FPCB stills.

Section snippets

Experimental setup and procedure

This work mainly consists of two systems namely conventional single slope single basin and flat plate collector basin stills. Both stills consist of a wooden box made by plywood having four sides with dimensions of 1.1 × 1.1 m2 and thickness of 0.025 m. The outer sides of the wooden box are covered by the sheet metal. The basins of both stills are fabricated with a mild steel plate and effective basin area of 1 m2. The basins are placed in the inner side of the plywood box. The space available

Efficiency of solar still

The daily efficiency, η, is obtained by the summation of the hourly condensate production m, multiplied by the latent heat hfg; hence the result is divided by the daily average solar radiation Is over the whole area A of the device:η=m×hfgA×Is.

Uncertainty analysis

The uncertainty analysis for the measuring instruments such as thermocouples, solarimeter, anemometer, measuring jar and daily efficiency is calculated from Eqs. (2), (3) as given by Rahbar and Esfahani [28] and experimental errors are calculated as given by Velmurugan et al. [29].u=a3uη=ηum2m2+uIs2Is212

The daily uncertainty for efficiency is varied from 0.03 to 0.04% and 0.05 to 0.06% for conventional and FPCB stills respectively. Table 1 shows the range, accuracy and uncertainty of various

Results and discussion

Each experiment is conducted for three times to investigate the performance of the still at different days. The deviation in operational conditions (solar intensity and ambient temperature) within 10% is chosen for comparison and discussion purposes.

Fig. 3 compares the performance of conventional and FPCB stills at 1 cm depth with various environmental and still parameters. It can be noticed that the distillate output and hourly efficiency of the FPCB still is always higher than those of the

Economic analysis

Economic analysis is used to estimate the unit cost of the distillated water by stills. The unit cost of the distilled water can be calculated by using Eq. (2). Here the average year around productivity of the solar still is taken about 60% of its daily original productivity, due to the year around variation in climatic condition. The solving methodology and other parameters used for the economic analysis are provided in Appendix A [31], [32].Cdw=TACM

Table 3 shows the economic analysis of

Conclusion

A flat plate collector basin still and conventional basin still are fabricated and tested under local climatic condition with different modifications in the basin. Jute cloth and black gravels are used in the basin to improve the evaporation rate and heat capacity of the still. Result indicates that the FPCB still has higher evaporation rate than the conventional basin still. The effect of extended surface and preheated water supply increases the distillate of the FPCB still about 60% than that

Nomenclature

    AMC

    annual maintenance cost, $

    ASV

    annual salvage value, $

    A

    area, m2

    a

    accuracy of instrument

    Cdw

    unit cost of distilled water, $/kg/m2

    CRF

    capital recovery factor

    FAC

    annualized capital cost, $

    FPCB

    flat plate collector basin

    H

    depth of water in measuring jar, m

    hfg

    enthalpy of evaporation at Tw, J/kg

    Is

    solar intensity, W/m2

    i

    interest rate, %

    M

    annual productivity, kg/m2

    m

    hourly productivity, kg/m2

    n

    number of life years of the system

    P

    capital cost, $

    Prod

    daily productivity, kg/day

    SFF

    sinking fund factor

    S

    salvage value, $

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